Horizontal eddy viscosity - Revision history

Taylorgasher: clarification on negative ESLM, added notes on internal flags

2020-07-06T15:23:05Z

clarification on negative ESLM, added notes on internal flags

← Older revision		Revision as of 11:23, 6 July 2020
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	</ref>. There are currently two slightly different ways of enabling this in the model. One is by specifying a negative value for <code>[[ESLM]]</code> in the [[fort.15 file]]. If this is done, the Smagorinsky turbulence closure is enabled and the absolute value of <code>ESLM</code> is used as the coefficient. Alternatively, the <code>Smag_Control</code> namelist can be used, e.g. by adding the following to the end of the fort.15 file <br/>		</ref>. There are currently two slightly different ways of enabling this in the model. One is by specifying a negative value for <code>[[ESLM]]</code> in the [[fort.15 file]]. If this is done, the Smagorinsky turbulence closure is enabled and the absolute value of <code>ESLM</code> is used as the coefficient. Alternatively, the <code>Smag_Control</code> namelist can be used, e.g. by adding the following to the end of the fort.15 file <br/>
	<code>&Smag_Control SMAG_LOWER_LIM=1.0e-8, SMAG_UPPER_LIM=100 /</code><br/>		<code>&Smag_Control SMAG_LOWER_LIM=1.0e-8, SMAG_UPPER_LIM=100 /</code><br/>
	Here again, the value of <code>ESLM</code> is taken as the coefficient. ~~This~~ also allows the user to optionally specify lower and upper bounds on the actual eddy viscosity determined via the Smagorinsky formulation. The values given in the above example of 1E-8 m<sup>2</sup>/s and 100 m<sup>2</sup>/s are the default values. There is no way to supply upper and lower bounds with the other method.		Here again, the absolute value of <code>ESLM</code> is taken as the coefficient, though a negative <code>ESLM</code> value isn't necessary if the namelist is used. The namelist also allows the user to optionally specify lower and upper bounds on the actual eddy viscosity determined via the Smagorinsky formulation. The values given in the above example of 1E-8 m<sup>2</sup>/s and 100 m<sup>2</sup>/s are the default values. There is no way to supply upper and lower bounds with the other method.

	If the [[fort.13_file#Eddy_Viscosity\|<code>average_horizontal_eddy_viscosity_in_sea_water_wrt_depth</code>]] nodal attribute is used when the Smagorinsky eddy viscosity is employed, then the nodal attribute values are taken to be the Smagorinsky coefficient values.		If the [[fort.13_file#Eddy_Viscosity\|<code>average_horizontal_eddy_viscosity_in_sea_water_wrt_depth</code>]] nodal attribute is used when the Smagorinsky eddy viscosity is employed, then the nodal attribute values are taken to be the Smagorinsky coefficient values.

	=== Usage Notes ===		=== Usage Notes ===
	The two methods currently result in different flags being set in the code, which have slightly inconsistent behavior. Specifically, the "negative <code>ESLM</code> method" results in a check being run at model initialization to verify that the default (Kolar-Gray) formulation of the lateral stresses in the GWCE is not in use, and the model errors out if it is. That check is not done in the namelist method. The lateral stress formulation is specified by the first digit in a [[IM#Six-digit_IM_Codes\|six-digit IM code]] being set to 1. It is also set for any regular IM value for 2D ADCIRC. Mathematically, the Kolar-Gray formulation of the lateral stress does not take into account a derivative of the eddy viscosity terms, meaning it is technically inconsistent with a Smagorinsky turbulence closure model. However, it is not clear whether the time derivative term is ever large enough to be consequential, and therefore merit departure from ADCIRC's default GWCE formulation.		The two methods currently result in different flags being set in the code, which have slightly inconsistent behavior. Specifically, the "negative <code>ESLM</code> method" sets <code>CSmag_Eh=.TRUE.</code> and results in a check being run at model initialization to verify that the default (Kolar-Gray) formulation of the lateral stresses in the GWCE is not in use, and the model errors out if it is. That check is not done in the namelist method, which sets <code>Smag_Comp_Flag=.TRUE.</code>. The lateral stress formulation is specified by the first digit in a [[IM#Six-digit_IM_Codes\|six-digit IM code]] being set to 1. It is also set for any regular IM value for 2D ADCIRC. Mathematically, the Kolar-Gray formulation of the lateral stress does not take into account a derivative of the eddy viscosity terms, meaning it is technically inconsistent with a Smagorinsky turbulence closure model. However, it is not clear whether the time derivative term is ever large enough to be consequential, and therefore merit departure from ADCIRC's default GWCE formulation.

	==References==		==References==
	<references />		<references />

Taylorgasher: /* Smagorinsky Eddy Viscosity */ clarification

2020-01-16T15:54:45Z

Smagorinsky Eddy Viscosity: clarification

← Older revision		Revision as of 11:54, 16 January 2020
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	== Smagorinsky Eddy Viscosity ==		== Smagorinsky Eddy Viscosity ==
	ADCIRC also allows a [https://en.wikipedia.org/wiki/Turbulence_modeling#Smagorinsky_model_for_the_sub-grid_scale_eddy_viscosity Smagorinsky-type] turbulence closure model<ref name=smag1>Smagorinsky, J. “General Circulation Experiments with the Primitive Equations.” Monthly Weather Review 91, no. 3 (March 1, 1963): 99–164. https://doi.org/10.1175/1520-0493(1963)091<0099:GCEWTP>2.3.CO;2.		ADCIRC also allows a [https://en.wikipedia.org/wiki/Turbulence_modeling#Smagorinsky_model_for_the_sub-grid_scale_eddy_viscosity Smagorinsky-type] turbulence closure model<ref name=smag1>Smagorinsky, J. “General Circulation Experiments with the Primitive Equations.” Monthly Weather Review 91, no. 3 (March 1, 1963): 99–164. https://doi.org/10.1175/1520-0493(1963)091<0099:GCEWTP>2.3.CO;2.
	</ref>. There are currently two slightly different ways of enabling this in the model. One is by specifying a negative value for <code>[[ESLM]]</code> in the [[fort.15 file]]. If this is done, the Smagorinsky turbulence closure is enabled and the absolute value of <code>ESLM</code> is used as the coefficient. Alternatively, the <code>Smag_Control</code> namelist can be used, e.g.<br/>		</ref>. There are currently two slightly different ways of enabling this in the model. One is by specifying a negative value for <code>[[ESLM]]</code> in the [[fort.15 file]]. If this is done, the Smagorinsky turbulence closure is enabled and the absolute value of <code>ESLM</code> is used as the coefficient. Alternatively, the <code>Smag_Control</code> namelist can be used, e.g. by adding the following to the end of the fort.15 file <br/>
	<code>&Smag_Control SMAG_LOWER_LIM=1.0e-8, SMAG_UPPER_LIM=100 /</code><br/>		<code>&Smag_Control SMAG_LOWER_LIM=1.0e-8, SMAG_UPPER_LIM=100 /</code><br/>
	~~When this is done~~, the value of <code>ESLM</code> is taken as the coefficient. This also allows the user to optionally specify lower and upper bounds on the actual eddy viscosity determined via the Smagorinsky formulation. The values given in the above example of 1E-8 m<sup>2</sup>/s and 100 m<sup>2</sup>/s are the default values. There is no way to supply upper and lower bounds with the other method.		Here again, the value of <code>ESLM</code> is taken as the coefficient. This also allows the user to optionally specify lower and upper bounds on the actual eddy viscosity determined via the Smagorinsky formulation. The values given in the above example of 1E-8 m<sup>2</sup>/s and 100 m<sup>2</sup>/s are the default values. There is no way to supply upper and lower bounds with the other method.

	If the [[fort.13_file#Eddy_Viscosity\|<code>average_horizontal_eddy_viscosity_in_sea_water_wrt_depth</code>]] nodal attribute is used when the Smagorinsky eddy viscosity is employed, then the nodal attribute values are taken to be the Smagorinsky coefficient values.		If the [[fort.13_file#Eddy_Viscosity\|<code>average_horizontal_eddy_viscosity_in_sea_water_wrt_depth</code>]] nodal attribute is used when the Smagorinsky eddy viscosity is employed, then the nodal attribute values are taken to be the Smagorinsky coefficient values.

Taylorgasher: /* Smagorinsky Eddy Viscosity */ typo

2019-10-01T13:25:44Z

Smagorinsky Eddy Viscosity: typo

← Older revision		Revision as of 09:25, 1 October 2019
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	ADCIRC also allows a [https://en.wikipedia.org/wiki/Turbulence_modeling#Smagorinsky_model_for_the_sub-grid_scale_eddy_viscosity Smagorinsky-type] turbulence closure model<ref name=smag1>Smagorinsky, J. “General Circulation Experiments with the Primitive Equations.” Monthly Weather Review 91, no. 3 (March 1, 1963): 99–164. https://doi.org/10.1175/1520-0493(1963)091<0099:GCEWTP>2.3.CO;2.		ADCIRC also allows a [https://en.wikipedia.org/wiki/Turbulence_modeling#Smagorinsky_model_for_the_sub-grid_scale_eddy_viscosity Smagorinsky-type] turbulence closure model<ref name=smag1>Smagorinsky, J. “General Circulation Experiments with the Primitive Equations.” Monthly Weather Review 91, no. 3 (March 1, 1963): 99–164. https://doi.org/10.1175/1520-0493(1963)091<0099:GCEWTP>2.3.CO;2.
	</ref>. There are currently two slightly different ways of enabling this in the model. One is by specifying a negative value for <code>[[ESLM]]</code> in the [[fort.15 file]]. If this is done, the Smagorinsky turbulence closure is enabled and the absolute value of <code>ESLM</code> is used as the coefficient. Alternatively, the <code>Smag_Control</code> namelist can be used, e.g.<br/>		</ref>. There are currently two slightly different ways of enabling this in the model. One is by specifying a negative value for <code>[[ESLM]]</code> in the [[fort.15 file]]. If this is done, the Smagorinsky turbulence closure is enabled and the absolute value of <code>ESLM</code> is used as the coefficient. Alternatively, the <code>Smag_Control</code> namelist can be used, e.g.<br/>
	<code>&Smag_Control SMAG_LOWER_LIM=1.0d-8, SMAG_UPPER_LIM=100 /</code><br/>		<code>&Smag_Control SMAG_LOWER_LIM=1.0e-8, SMAG_UPPER_LIM=100 /</code><br/>
	When this is done, the value of <code>ESLM</code> is taken as the coefficient. This also allows the user to optionally specify lower and upper bounds on the actual eddy viscosity determined via the Smagorinsky formulation. The values given in the above example of 1E-8 m<sup>2</sup>/s and 100 m<sup>2</sup>/s are the default values. There is no way to supply upper and lower bounds with the other method.		When this is done, the value of <code>ESLM</code> is taken as the coefficient. This also allows the user to optionally specify lower and upper bounds on the actual eddy viscosity determined via the Smagorinsky formulation. The values given in the above example of 1E-8 m<sup>2</sup>/s and 100 m<sup>2</sup>/s are the default values. There is no way to supply upper and lower bounds with the other method.

Taylorgasher: /* Smagorinsky Eddy Viscosity */ revised language on derivative...it appears both spatial and temporal derivatives of eddy viscosity are neglected

2019-09-28T17:29:57Z

Smagorinsky Eddy Viscosity: revised language on derivative...it appears both spatial and temporal derivatives of eddy viscosity are neglected

← Older revision		Revision as of 13:29, 28 September 2019
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	=== Usage Notes ===		=== Usage Notes ===
	The two methods currently result in different flags being set in the code, which have slightly inconsistent behavior. Specifically, the "negative <code>ESLM</code> method" results in a check being run at model initialization to verify that the default (Kolar-Gray) formulation of the lateral stresses in the GWCE is not in use, and the model errors out if it is. That check is not done in the namelist method. The lateral stress formulation is specified by the first digit in a [[IM#Six-digit_IM_Codes\|six-digit IM code]] being set to 1. It is also set for any regular IM value for 2D ADCIRC. Mathematically, the Kolar-Gray formulation of the lateral stress does not take into account a ~~time~~ derivative of the eddy viscosity terms, meaning it is technically inconsistent with a Smagorinsky turbulence closure model. However, it is not clear whether the time derivative term is ever large enough to be consequential, and therefore merit departure from ADCIRC's default GWCE formulation.		The two methods currently result in different flags being set in the code, which have slightly inconsistent behavior. Specifically, the "negative <code>ESLM</code> method" results in a check being run at model initialization to verify that the default (Kolar-Gray) formulation of the lateral stresses in the GWCE is not in use, and the model errors out if it is. That check is not done in the namelist method. The lateral stress formulation is specified by the first digit in a [[IM#Six-digit_IM_Codes\|six-digit IM code]] being set to 1. It is also set for any regular IM value for 2D ADCIRC. Mathematically, the Kolar-Gray formulation of the lateral stress does not take into account a derivative of the eddy viscosity terms, meaning it is technically inconsistent with a Smagorinsky turbulence closure model. However, it is not clear whether the time derivative term is ever large enough to be consequential, and therefore merit departure from ADCIRC's default GWCE formulation.

	==References==		==References==
	<references />		<references />

Taylorgasher: /* Smagorinsky Eddy Viscosity */

2019-09-22T05:51:29Z

Smagorinsky Eddy Viscosity

← Older revision		Revision as of 01:51, 22 September 2019
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	When this is done, the value of <code>ESLM</code> is taken as the coefficient. This also allows the user to optionally specify lower and upper bounds on the actual eddy viscosity determined via the Smagorinsky formulation. The values given in the above example of 1E-8 m<sup>2</sup>/s and 100 m<sup>2</sup>/s are the default values. There is no way to supply upper and lower bounds with the other method.		When this is done, the value of <code>ESLM</code> is taken as the coefficient. This also allows the user to optionally specify lower and upper bounds on the actual eddy viscosity determined via the Smagorinsky formulation. The values given in the above example of 1E-8 m<sup>2</sup>/s and 100 m<sup>2</sup>/s are the default values. There is no way to supply upper and lower bounds with the other method.

	If the [[fort.13_file#Eddy_Viscosity\|<code>average_horizontal_eddy_viscosity_in_sea_water_wrt_depth</code>]] nodal attribute is used when the Smagorinsky~~-type~~ eddy viscosity is employed, then the nodal attribute values are taken to be the Smagorinsky coefficient values.		If the [[fort.13_file#Eddy_Viscosity\|<code>average_horizontal_eddy_viscosity_in_sea_water_wrt_depth</code>]] nodal attribute is used when the Smagorinsky eddy viscosity is employed, then the nodal attribute values are taken to be the Smagorinsky coefficient values.

	=== Usage Notes ===		=== Usage Notes ===

Taylorgasher: /* Smagorinsky Eddy Viscosity */

2019-09-22T05:51:03Z

Smagorinsky Eddy Viscosity

← Older revision		Revision as of 01:51, 22 September 2019
Line 13:		Line 13:
	When this is done, the value of <code>ESLM</code> is taken as the coefficient. This also allows the user to optionally specify lower and upper bounds on the actual eddy viscosity determined via the Smagorinsky formulation. The values given in the above example of 1E-8 m<sup>2</sup>/s and 100 m<sup>2</sup>/s are the default values. There is no way to supply upper and lower bounds with the other method.		When this is done, the value of <code>ESLM</code> is taken as the coefficient. This also allows the user to optionally specify lower and upper bounds on the actual eddy viscosity determined via the Smagorinsky formulation. The values given in the above example of 1E-8 m<sup>2</sup>/s and 100 m<sup>2</sup>/s are the default values. There is no way to supply upper and lower bounds with the other method.

	If the [[fort.13_file#Eddy_Viscosity\|<code>average_horizontal_eddy_viscosity_in_sea_water_wrt_depth</code>]] nodal attribute is used when Smagorinsky-type eddy viscosity is employed, then the nodal attribute values are taken to be the Smagorinsky coefficient values.		If the [[fort.13_file#Eddy_Viscosity\|<code>average_horizontal_eddy_viscosity_in_sea_water_wrt_depth</code>]] nodal attribute is used when the Smagorinsky-type eddy viscosity is employed, then the nodal attribute values are taken to be the Smagorinsky coefficient values.

	=== Usage Notes ===		=== Usage Notes ===
	The two methods currently result in different flags being set in the code, which have slightly inconsistent behavior. Specifically, the "negative <code>ESLM</code> method" results in a check being run at model initialization to verify that the default (Kolar-Gray) formulation of the lateral stresses in the GWCE is not in use, and the model errors out if it is. That check is not done in the namelist method. The lateral stress formulation is specified by the first digit in a [[IM#Six-digit_IM_Codes\|six-digit IM code]] being set to 1. It is also set for any regular IM value for 2D ADCIRC. Mathematically, the Kolar-Gray formulation of the lateral stress does not take into account a time derivative of the eddy viscosity terms, meaning it is technically inconsistent with a Smagorinsky turbulence closure model. However, it is not clear whether the time derivative term is ever large enough to be consequential, and therefore merit departure from ADCIRC's default GWCE formulation.		The two methods currently result in different flags being set in the code, which have slightly inconsistent behavior. Specifically, the "negative <code>ESLM</code> method" results in a check being run at model initialization to verify that the default (Kolar-Gray) formulation of the lateral stresses in the GWCE is not in use, and the model errors out if it is. That check is not done in the namelist method. The lateral stress formulation is specified by the first digit in a [[IM#Six-digit_IM_Codes\|six-digit IM code]] being set to 1. It is also set for any regular IM value for 2D ADCIRC. Mathematically, the Kolar-Gray formulation of the lateral stress does not take into account a time derivative of the eddy viscosity terms, meaning it is technically inconsistent with a Smagorinsky turbulence closure model. However, it is not clear whether the time derivative term is ever large enough to be consequential, and therefore merit departure from ADCIRC's default GWCE formulation.

	==References==		==References==
	<references />		<references />

Taylorgasher: Added how nodal attrib works w/Smag, per email from William

2019-09-22T05:50:39Z

Added how nodal attrib works w/Smag, per email from William

← Older revision		Revision as of 01:50, 22 September 2019
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	<code>&Smag_Control SMAG_LOWER_LIM=1.0d-8, SMAG_UPPER_LIM=100 /</code><br/>		<code>&Smag_Control SMAG_LOWER_LIM=1.0d-8, SMAG_UPPER_LIM=100 /</code><br/>
	When this is done, the value of <code>ESLM</code> is taken as the coefficient. This also allows the user to optionally specify lower and upper bounds on the actual eddy viscosity determined via the Smagorinsky formulation. The values given in the above example of 1E-8 m<sup>2</sup>/s and 100 m<sup>2</sup>/s are the default values. There is no way to supply upper and lower bounds with the other method.		When this is done, the value of <code>ESLM</code> is taken as the coefficient. This also allows the user to optionally specify lower and upper bounds on the actual eddy viscosity determined via the Smagorinsky formulation. The values given in the above example of 1E-8 m<sup>2</sup>/s and 100 m<sup>2</sup>/s are the default values. There is no way to supply upper and lower bounds with the other method.

			If the [[fort.13_file#Eddy_Viscosity\|<code>average_horizontal_eddy_viscosity_in_sea_water_wrt_depth</code>]] nodal attribute is used when Smagorinsky-type eddy viscosity is employed, then the nodal attribute values are taken to be the Smagorinsky coefficient values.

	=== Usage Notes ===		=== Usage Notes ===

Wpringle at 22:51, 19 September 2019

2019-09-19T22:51:42Z

← Older revision		Revision as of 18:51, 19 September 2019
Line 1:		Line 1:
	'''Horizontal eddy viscosity''' is a term in the momentum equations to cover the well-known turbulence closure problem. For background on the problem, see, e.g. the [http://glossary.ametsoc.org/wiki/Turbulence_closure AMS glossary definition], the [https://en.wikipedia.org/wiki/Turbulence_modeling Wikipedia entry on turbulence modeling], or a fluid mechanics textbook such as Kundu's<ref name=kundu>Pijush K. Kundu, Ira M. Cohen, David R. Dowling (2012). Fluid Mechanics.</ref>. Readers unfamiliar with these concepts should note that, contrary to the name, eddy viscosity is unrelated to true viscosity, even though the units (length squared over time) are the same. Typically, one should expect eddy viscosity to be many orders of magnitude larger than the viscosity of actual water (around 1x10<sup>-6</sup> meters<sup>2</sup>/second for seawater). In ADCIRC, ~~horizontal eddy viscosity~~ can ~~be treated via~~ either a ~~manually specified~~ eddy viscosity or a Smagorinsky~~-type~~ eddy viscosity. In ~~all~~ cases, the ~~parameter is useful both for dealing with the true turbulence closure problem and for aiding with numerical stability~~.		'''Horizontal eddy viscosity''' is a term in the momentum equations to cover the well-known turbulence closure problem. The term is also useful numerically for increased stability. For background on the turbulence closure problem, see, e.g. the [http://glossary.ametsoc.org/wiki/Turbulence_closure AMS glossary definition], the [https://en.wikipedia.org/wiki/Turbulence_modeling Wikipedia entry on turbulence modeling], or a fluid mechanics textbook such as Kundu's<ref name=kundu>Pijush K. Kundu, Ira M. Cohen, David R. Dowling (2012). Fluid Mechanics.</ref>. Readers unfamiliar with these concepts should note that, contrary to the name, eddy viscosity is unrelated to true viscosity, even though the units (length squared over time) are the same. Typically, one should expect eddy viscosity to be many orders of magnitude larger than the viscosity of actual water (around 1x10<sup>-6</sup> meters<sup>2</sup>/second for seawater). In ADCIRC, the user can either specify a constant in time horizontal eddy viscosity or employ the Smagorinsky model to update the horizontal eddy viscosity based on local flow conditions with each time step. In both cases the horizontal eddy viscosity or Smagorinsky coefficient can be either spatially constant or spatially varying.

	== ~~Manually~~ Specified Eddy Viscosity ==		== User-Specified Constant in Time Eddy Viscosity ==
	A spatially constant eddy viscosity can be supplied via the <code>[[ESLM]]</code> parameter in the [[fort.15 file]]. Alternatively, the [[fort.13_file#Eddy_Viscosity\|<code>average_horizontal_eddy_viscosity_in_sea_water_wrt_depth</code>]] nodal attribute permits a spatially variable eddy viscosity.		A spatially constant eddy viscosity can be supplied via the <code>[[ESLM]]</code> parameter in the [[fort.15 file]]. Alternatively, the [[fort.13_file#Eddy_Viscosity\|<code>average_horizontal_eddy_viscosity_in_sea_water_wrt_depth</code>]] nodal attribute permits a spatially variable eddy viscosity.

Taylorgasher: added stuff, I think this is mostly done now...

2019-09-19T22:32:38Z

added stuff, I think this is mostly done now...

← Older revision		Revision as of 18:32, 19 September 2019
Line 13:		Line 13:
	When this is done, the value of <code>ESLM</code> is taken as the coefficient. This also allows the user to optionally specify lower and upper bounds on the actual eddy viscosity determined via the Smagorinsky formulation. The values given in the above example of 1E-8 m<sup>2</sup>/s and 100 m<sup>2</sup>/s are the default values. There is no way to supply upper and lower bounds with the other method.		When this is done, the value of <code>ESLM</code> is taken as the coefficient. This also allows the user to optionally specify lower and upper bounds on the actual eddy viscosity determined via the Smagorinsky formulation. The values given in the above example of 1E-8 m<sup>2</sup>/s and 100 m<sup>2</sup>/s are the default values. There is no way to supply upper and lower bounds with the other method.

	Usage Notes		=== Usage Notes ===
	The two methods currently result in different flags being set in the code, which have inconsistent behavior. ~~Most notably~~, the "negative <code>ESLM</code> method" results in a check being run at model initialization to verify that the Kolar-Gray formulation of the lateral stresses in the GWCE is not in use, and the ~~code~~ errors out if it is. ~~This choice~~ is specified by the first digit in a six-digit IM code being set to 1.		The two methods currently result in different flags being set in the code, which have slightly inconsistent behavior. Specifically, the "negative <code>ESLM</code> method" results in a check being run at model initialization to verify that the default (Kolar-Gray) formulation of the lateral stresses in the GWCE is not in use, and the model errors out if it is. That check is not done in the namelist method. The lateral stress formulation is specified by the first digit in a [[IM#Six-digit_IM_Codes\|six-digit IM code]] being set to 1. It is also set for any regular IM value for 2D ADCIRC. Mathematically, the Kolar-Gray formulation of the lateral stress does not take into account a time derivative of the eddy viscosity terms, meaning it is technically inconsistent with a Smagorinsky turbulence closure model. However, it is not clear whether the time derivative term is ever large enough to be consequential, and therefore merit departure from ADCIRC's default GWCE formulation.

	~~digit 1~~ of ~~IM==1 isn~~'~~t allowed~~

	==References==		==References==
	<references />		<references />

Taylorgasher: Created page, work-in-progress

2019-09-19T22:21:15Z

Created page, work-in-progress

New page

'''Horizontal eddy viscosity''' is a term in the momentum equations to cover the well-known turbulence closure problem. For background on the problem, see, e.g. the [http://glossary.ametsoc.org/wiki/Turbulence_closure AMS glossary definition], the [https://en.wikipedia.org/wiki/Turbulence_modeling Wikipedia entry on turbulence modeling], or a fluid mechanics textbook such as Kundu's<ref name=kundu>Pijush K. Kundu, Ira M. Cohen, David R. Dowling (2012). Fluid Mechanics.</ref>. Readers unfamiliar with these concepts should note that, contrary to the name, eddy viscosity is unrelated to true viscosity, even though the units (length squared over time) are the same. Typically, one should expect eddy viscosity to be many orders of magnitude larger than the viscosity of actual water (around 1x10-6 meters2/second for seawater). In ADCIRC, horizontal eddy viscosity can be treated via either a manually specified eddy viscosity or a Smagorinsky-type eddy viscosity. In all cases, the parameter is useful both for dealing with the true turbulence closure problem and for aiding with numerical stability.

== Manually Specified Eddy Viscosity ==
A spatially constant eddy viscosity can be supplied via the <code>[[ESLM]]</code> parameter in the [[fort.15 file]]. Alternatively, the [[fort.13_file#Eddy_Viscosity|<code>average_horizontal_eddy_viscosity_in_sea_water_wrt_depth</code>]] nodal attribute permits a spatially variable eddy viscosity.

=== Usage Notes ===
Commonly used values range from 1 to 50 meters2/second, with 10 being a good starting point for many coastal ocean modeling scenarios. By the nature of turbulence closure, one might expect the value to be smaller as mesh resolution increases. However, this may not always be the case since areas with high resolution may also be more turbulent. In particular, some modelers use a larger value in overland areas, which can also improve stability. However, specifying a value that is too large can induce instabilities, as well.

== Smagorinsky Eddy Viscosity ==
ADCIRC also allows a [https://en.wikipedia.org/wiki/Turbulence_modeling#Smagorinsky_model_for_the_sub-grid_scale_eddy_viscosity Smagorinsky-type] turbulence closure model<ref name=smag1>Smagorinsky, J. “General Circulation Experiments with the Primitive Equations.” Monthly Weather Review 91, no. 3 (March 1, 1963): 99–164. https://doi.org/10.1175/1520-0493(1963)091<0099:GCEWTP>2.3.CO;2.
</ref>. There are currently two slightly different ways of enabling this in the model. One is by specifying a negative value for <code>[[ESLM]]</code> in the [[fort.15 file]]. If this is done, the Smagorinsky turbulence closure is enabled and the absolute value of <code>ESLM</code> is used as the coefficient. Alternatively, the <code>Smag_Control</code> namelist can be used, e.g. 
<code>&Smag_Control SMAG_LOWER_LIM=1.0d-8, SMAG_UPPER_LIM=100 /</code> 
When this is done, the value of <code>ESLM</code> is taken as the coefficient. This also allows the user to optionally specify lower and upper bounds on the actual eddy viscosity determined via the Smagorinsky formulation. The values given in the above example of 1E-8 m2/s and 100 m2/s are the default values. There is no way to supply upper and lower bounds with the other method.

Usage Notes
The two methods currently result in different flags being set in the code, which have inconsistent behavior. Most notably, the "negative <code>ESLM</code> method" results in a check being run at model initialization to verify that the Kolar-Gray formulation of the lateral stresses in the GWCE is not in use, and the code errors out if it is. This choice is specified by the first digit in a six-digit IM code being set to 1.

digit 1 of IM==1 isn't allowed

==References==
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