1.  Eca, Luis; Saraiva, Goncalo; Vaz, Guilherme; Abreu, Hugo: The PROS and CONS of Wall Functions. Proceedings of the ASME 2015 34st International Conference on Ocean, Offshore and Arctic Engineering, May 31stJune 5th, St. John’s, Canada, 2015. (Type: Conference  Abstract  Links  BibTeX) @conference{OMAE2015_Eca_WallFunctions, title = {The PROS and CONS of Wall Functions}, author = {Luis Eca and Goncalo Saraiva and Guilherme Vaz and Hugo Abreu}, url = {http://www.refresco.org/?wpdmpro=2015omae41518_ecavaz_et_al_wallfunctionspdf http://www.asmeconferences.org/}, year = {2015}, date = {20150601}, booktitle = {Proceedings of the ASME 2015 34st International Conference on Ocean, Offshore and Arctic Engineering, May 31stJune 5th, St. John’s, Canada}, journal = {Proceedings of the ASME 2015 34st International Conference on Ocean, Offshore and Arctic Engineering, May 31stJune 5th, 2015, St. John’s, Canada}, abstract = {Averaged NavierStokes (RANS) equations have become an engineering tool used on a daily basis. One of the main goals of such calculations is to determine friction forces, which are a consequence of the shearstress at solid walls. In RANS (and other more sophisticated mathematical models), there are two main approaches for the determination of the shearstress at a wall: direct application of the noslip condition, i.e. the velocity gradient is determined directly at the surface; wall functions which determine the shearstress at the wall from semiempirical equations applicable up to the outer edge of the socalled ”wall layer/log layer”. Although the first option is physically preferable, its numerical requirements may lead to iterative convergence problems and/or excessive calculation times. Therefore, especially at high Reynolds numbers, it is not unusual to use the latter approach. In this paper we discuss the advantages and disadvantages of wallfunction boundary conditions. To this end we have calculated the flow around a flat plate, conventional and laminar airfoils and a circular cylinder. The influence of the location where wall functions are applied (distance to the wall) and the effect of the Reynolds number (ranging from model to full scale applications) are discussed. Griding requirements for wallfunction boundary conditions are also addressed. The results obtained with wall functions are compared with those obtained from the direct application of the no slip at the wall. The results obtained in this study show that the use of wall functions in viscous flow calculations may be justifiable or completely unacceptable depending on the flow conditions. Furthermore, it is also shown that wallfunction boundary conditions also require clustering of grid nodes close to the wall, but obviously less demanding than the direct application of no slip condition.}, keywords = {}, pubstate = {published}, tppubtype = {conference} } Averaged NavierStokes (RANS) equations have become an engineering tool used on a daily basis. One of the main goals of such calculations is to determine friction forces, which are a consequence of the shearstress at solid walls. In RANS (and other more sophisticated mathematical models), there are two main approaches for the determination of the shearstress at a wall: direct application of the noslip condition, i.e. the velocity gradient is determined directly at the surface; wall functions which determine the shearstress at the wall from semiempirical equations applicable up to the outer edge of the socalled ”wall layer/log layer”. Although the first option is physically preferable, its numerical requirements may lead to iterative convergence problems and/or excessive calculation times. Therefore, especially at high Reynolds numbers, it is not unusual to use the latter approach. In this paper we discuss the advantages and disadvantages of wallfunction boundary conditions. To this end we have calculated the flow around a flat plate, conventional and laminar airfoils and a circular cylinder. The influence of the location where wall functions are applied (distance to the wall) and the effect of the Reynolds number (ranging from model to full scale applications) are discussed. Griding requirements for wallfunction boundary conditions are also addressed. The results obtained with wall functions are compared with those obtained from the direct application of the no slip at the wall. The results obtained in this study show that the use of wall functions in viscous flow calculations may be justifiable or completely unacceptable depending on the flow conditions. Furthermore, it is also shown that wallfunction boundary conditions also require clustering of grid nodes close to the wall, but obviously less demanding than the direct application of no slip condition. 
2.  Saraiva, Goncalo: Solution of Flows Around Airfoils Using RANS with WallFunctions. IST, Lisbon, Portugal, 2014. (Type: Masters Thesis  Abstract  Links  BibTeX) @mastersthesis{2014Msc_Thesis_GoncaloSaraiva, title = {Solution of Flows Around Airfoils Using RANS with WallFunctions}, author = {Goncalo Saraiva}, url = { http://www.refresco.org/?wpdmpro=2014msc_thesis_goncalosaraivapdf}, year = {2014}, date = {20141024}, school = {IST, Lisbon, Portugal}, abstract = {The calculation of the friction forces is essential in hydrodynamic and offshore applications. However, the high gradients that exist in nearwall regions require the use of one of the following approaches: grids that are very fine near the wall to calculate the wall shearstress directly from its definition; or wallfunctions (WF) to calculate indirectly the wall shearstress and provide boundary conditions for the variables of the turbulence models. The objective of this thesis is to assess the validity of WF boundary conditions for the calculation of friction and pressure coefficients, as well as aerodynamic forces coefficients of a conventional and a laminar airfoil. The ReFRESCO solver was used to solve the RANS equations with the SST version of the eddyviscosity turbulence model. The main conclusions obtained were: WF can yield acceptable results if the Reynolds number is high enough to promote transition near the leading edge; if the laminar part of the flow is significant, the results are not realistic because WF lead to a fully turbulent flow; the results for the pressure and lift coefficient are always better than for friction and drag coefficients due to the direct connection of the wall shearstress with the last two; last but not least, the results of the WF approach are strongly dependent on the location of the first interior grid node, even at high Reynolds number.}, keywords = {}, pubstate = {published}, tppubtype = {mastersthesis} } The calculation of the friction forces is essential in hydrodynamic and offshore applications. However, the high gradients that exist in nearwall regions require the use of one of the following approaches: grids that are very fine near the wall to calculate the wall shearstress directly from its definition; or wallfunctions (WF) to calculate indirectly the wall shearstress and provide boundary conditions for the variables of the turbulence models. The objective of this thesis is to assess the validity of WF boundary conditions for the calculation of friction and pressure coefficients, as well as aerodynamic forces coefficients of a conventional and a laminar airfoil. The ReFRESCO solver was used to solve the RANS equations with the SST version of the eddyviscosity turbulence model. The main conclusions obtained were: WF can yield acceptable results if the Reynolds number is high enough to promote transition near the leading edge; if the laminar part of the flow is significant, the results are not realistic because WF lead to a fully turbulent flow; the results for the pressure and lift coefficient are always better than for friction and drag coefficients due to the direct connection of the wall shearstress with the last two; last but not least, the results of the WF approach are strongly dependent on the location of the first interior grid node, even at high Reynolds number. 
3.  Manzke, Manuel: Using FreSCo for the Determination of Frictional Forces. TUHH, Hamburg, Germany, 2007. (Type: Masters Thesis  Links  BibTeX) @mastersthesis{2007Stage_ManuelManzke, title = {Using FreSCo for the Determination of Frictional Forces}, author = {Manuel Manzke}, url = {http://www.refresco.org/?wpdmpro=2007stage_manuelmanzkepdf}, year = {2007}, date = {20070504}, school = {TUHH, Hamburg, Germany}, keywords = {}, pubstate = {published}, tppubtype = {mastersthesis} } 
2015 

Eca, Luis; Saraiva, Goncalo; Vaz, Guilherme; Abreu, Hugo The PROS and CONS of Wall Functions Conference Proceedings of the ASME 2015 34st International Conference on Ocean, Offshore and Arctic Engineering, May 31stJune 5th, St. John’s, Canada, 2015. Abstract  Links  BibTeX  Tags: Cylinder, Flatplate, Foils, RANS, SST, URANS, Validation, Verification, Wallfunctions, yplus @conference{OMAE2015_Eca_WallFunctions, title = {The PROS and CONS of Wall Functions}, author = {Luis Eca and Goncalo Saraiva and Guilherme Vaz and Hugo Abreu}, url = {http://www.refresco.org/?wpdmpro=2015omae41518_ecavaz_et_al_wallfunctionspdf http://www.asmeconferences.org/}, year = {2015}, date = {20150601}, booktitle = {Proceedings of the ASME 2015 34st International Conference on Ocean, Offshore and Arctic Engineering, May 31stJune 5th, St. John’s, Canada}, journal = {Proceedings of the ASME 2015 34st International Conference on Ocean, Offshore and Arctic Engineering, May 31stJune 5th, 2015, St. John’s, Canada}, abstract = {Averaged NavierStokes (RANS) equations have become an engineering tool used on a daily basis. One of the main goals of such calculations is to determine friction forces, which are a consequence of the shearstress at solid walls. In RANS (and other more sophisticated mathematical models), there are two main approaches for the determination of the shearstress at a wall: direct application of the noslip condition, i.e. the velocity gradient is determined directly at the surface; wall functions which determine the shearstress at the wall from semiempirical equations applicable up to the outer edge of the socalled ”wall layer/log layer”. Although the first option is physically preferable, its numerical requirements may lead to iterative convergence problems and/or excessive calculation times. Therefore, especially at high Reynolds numbers, it is not unusual to use the latter approach. In this paper we discuss the advantages and disadvantages of wallfunction boundary conditions. To this end we have calculated the flow around a flat plate, conventional and laminar airfoils and a circular cylinder. The influence of the location where wall functions are applied (distance to the wall) and the effect of the Reynolds number (ranging from model to full scale applications) are discussed. Griding requirements for wallfunction boundary conditions are also addressed. The results obtained with wall functions are compared with those obtained from the direct application of the no slip at the wall. The results obtained in this study show that the use of wall functions in viscous flow calculations may be justifiable or completely unacceptable depending on the flow conditions. Furthermore, it is also shown that wallfunction boundary conditions also require clustering of grid nodes close to the wall, but obviously less demanding than the direct application of no slip condition.}, keywords = {Cylinder, Flatplate, Foils, RANS, SST, URANS, Validation, Verification, Wallfunctions, yplus}, pubstate = {published}, tppubtype = {conference} } Averaged NavierStokes (RANS) equations have become an engineering tool used on a daily basis. One of the main goals of such calculations is to determine friction forces, which are a consequence of the shearstress at solid walls. In RANS (and other more sophisticated mathematical models), there are two main approaches for the determination of the shearstress at a wall: direct application of the noslip condition, i.e. the velocity gradient is determined directly at the surface; wall functions which determine the shearstress at the wall from semiempirical equations applicable up to the outer edge of the socalled ”wall layer/log layer”. Although the first option is physically preferable, its numerical requirements may lead to iterative convergence problems and/or excessive calculation times. Therefore, especially at high Reynolds numbers, it is not unusual to use the latter approach. In this paper we discuss the advantages and disadvantages of wallfunction boundary conditions. To this end we have calculated the flow around a flat plate, conventional and laminar airfoils and a circular cylinder. The influence of the location where wall functions are applied (distance to the wall) and the effect of the Reynolds number (ranging from model to full scale applications) are discussed. Griding requirements for wallfunction boundary conditions are also addressed. The results obtained with wall functions are compared with those obtained from the direct application of the no slip at the wall. The results obtained in this study show that the use of wall functions in viscous flow calculations may be justifiable or completely unacceptable depending on the flow conditions. Furthermore, it is also shown that wallfunction boundary conditions also require clustering of grid nodes close to the wall, but obviously less demanding than the direct application of no slip condition.  
2014 

Saraiva, Goncalo Solution of Flows Around Airfoils Using RANS with WallFunctions Masters Thesis IST, Lisbon, Portugal, 2014. Abstract  Links  BibTeX  Tags: Eppler, Foils, NACA 0012, RANS, SST, Validation, Verification, Wallfunctions @mastersthesis{2014Msc_Thesis_GoncaloSaraiva, title = {Solution of Flows Around Airfoils Using RANS with WallFunctions}, author = {Goncalo Saraiva}, url = { http://www.refresco.org/?wpdmpro=2014msc_thesis_goncalosaraivapdf}, year = {2014}, date = {20141024}, school = {IST, Lisbon, Portugal}, abstract = {The calculation of the friction forces is essential in hydrodynamic and offshore applications. However, the high gradients that exist in nearwall regions require the use of one of the following approaches: grids that are very fine near the wall to calculate the wall shearstress directly from its definition; or wallfunctions (WF) to calculate indirectly the wall shearstress and provide boundary conditions for the variables of the turbulence models. The objective of this thesis is to assess the validity of WF boundary conditions for the calculation of friction and pressure coefficients, as well as aerodynamic forces coefficients of a conventional and a laminar airfoil. The ReFRESCO solver was used to solve the RANS equations with the SST version of the eddyviscosity turbulence model. The main conclusions obtained were: WF can yield acceptable results if the Reynolds number is high enough to promote transition near the leading edge; if the laminar part of the flow is significant, the results are not realistic because WF lead to a fully turbulent flow; the results for the pressure and lift coefficient are always better than for friction and drag coefficients due to the direct connection of the wall shearstress with the last two; last but not least, the results of the WF approach are strongly dependent on the location of the first interior grid node, even at high Reynolds number.}, keywords = {Eppler, Foils, NACA 0012, RANS, SST, Validation, Verification, Wallfunctions}, pubstate = {published}, tppubtype = {mastersthesis} } The calculation of the friction forces is essential in hydrodynamic and offshore applications. However, the high gradients that exist in nearwall regions require the use of one of the following approaches: grids that are very fine near the wall to calculate the wall shearstress directly from its definition; or wallfunctions (WF) to calculate indirectly the wall shearstress and provide boundary conditions for the variables of the turbulence models. The objective of this thesis is to assess the validity of WF boundary conditions for the calculation of friction and pressure coefficients, as well as aerodynamic forces coefficients of a conventional and a laminar airfoil. The ReFRESCO solver was used to solve the RANS equations with the SST version of the eddyviscosity turbulence model. The main conclusions obtained were: WF can yield acceptable results if the Reynolds number is high enough to promote transition near the leading edge; if the laminar part of the flow is significant, the results are not realistic because WF lead to a fully turbulent flow; the results for the pressure and lift coefficient are always better than for friction and drag coefficients due to the direct connection of the wall shearstress with the last two; last but not least, the results of the WF approach are strongly dependent on the location of the first interior grid node, even at high Reynolds number.  
2007 

Manzke, Manuel Using FreSCo for the Determination of Frictional Forces Masters Thesis TUHH, Hamburg, Germany, 2007. Links  BibTeX  Tags: 1eq Menter, Drag, Flatplate, kepsilon, komega, SpalartAllmaras, SST, Turbulence Models, Validation, Verification, Wallfunctions @mastersthesis{2007Stage_ManuelManzke, title = {Using FreSCo for the Determination of Frictional Forces}, author = {Manuel Manzke}, url = {http://www.refresco.org/?wpdmpro=2007stage_manuelmanzkepdf}, year = {2007}, date = {20070504}, school = {TUHH, Hamburg, Germany}, keywords = {1eq Menter, Drag, Flatplate, kepsilon, komega, SpalartAllmaras, SST, Turbulence Models, Validation, Verification, Wallfunctions}, pubstate = {published}, tppubtype = {mastersthesis} } 