CFD stands for Computational Fluid Dynamics - the branch of mechanical engineering dealing with Fluid Flow and Heat Transfer. Whether you are a fresh graduate with engineering degree or a seasonsed professional with industry and design background, you will find plenty of technical stuffs on CFD analyses and CFD software (both commercial such as FLUENT, CFX and COMSOL as well as open-source Gmsh, OpenFOAM and ParaView. However, one must note that the term CFD is also used in commodity trading where it stands for Contracts for Difference.
How many of these names have you heard of? Click on the names to know more about them!
Y-plus: Estimation of first layer height near wall
Estimate height of the 1st layer height of the mesh as per recommended y+ value of the chosen solver, a better guess than choosing an arbitrary value.
Here is Excel version: Y-Plus and Boundary Layer Height Estimation using Excel. The default values have been specified to avoid arithmetic errors. Please ensure you specify all the values as per the operating conditions chosen in simulation. The default value of y+ is set = 30, appropriate for most high-Re turbulence models. In case a low-Re turbulence model such as SST is being used, change this value to 1 or maximum 5.
Select the type of geometry:
-
Select the working fluid:
[ - ]
Select mode of specifying velocity scale:
[ - ]
Specify velocity scale:
[m/s] or [m3/s] or [kg/s]
Specify cross-section area - ignored if velocity is specified:
[m2]
Select solver type: 1=Vertex-based (CFX), 2=Cell-centre based (Fluent):
[ - ]
Specify length scale of flow - hyd. dia. (Ducts) or length of the (Flat) plate:
[m]
Specify working temperature of the fluid:
[°C]
Specify 'absolute' working pressure (required in case of air only):
[bar]
Specify desired mean value of y+:
[ - ]
-
The calculations are based on empirical correlations and may still require iterations to meet Y+ requirements
The calculation is based on fully-developed regime. In developing regime, coarse mesh is required and hence the estimated mesh height will hold true for developing region also.
The calculation is applicable to TURBULENT flows only
The material properties (viscosity and density) are calculated based on specified fluid (currently available for air and water only) and its temperature.
The mesh requirements are different for vertex-based formulations (CFX) and cell-centred formulations (Fluent & STAR CCM+)
For identical mesh, boundary conditions and material properties, CFX will result in Y+ value approx. twice of that in Fluent, if Y+ ≥ 30 everywhere
If Y+ of flow results in a range 10 ~ 30, the value reported in CFX will not be closer to twice that reported in Fluent. This is because of "scalable" formulation in CFX.
For 2D geometries, a thickness of 1.0 [m] is assumed to estimate velocity from specified volumetric or mass flow
The steps required to estimate the first layer height is described below:
In order to demonstrate the effect of the cell-based and vertex-based scheme, simulations were performed using same mesh, material properties, solver and boundary conditions. The area-averaged value of Y-plus matched the observations described above. The mesh used is shown below.
The diameter is 100 [mm] and
the 1st layer height used in CFD simulation is 1.0 [mm]
The material properties used in CFX and Fluent are described below.
The turbulence model used was:
The boundary conditions at inlet and outlet are shown below.
A good level of convergence was achieved:
Conclusion:
The contour plots for the Y-plus are shown below. Based on the flow conditions and material properties, following are derived parameters:
Reynolds number = 65833
friction factor = 0.0197
friction velocity = 0.496 [m/s]
boundary layer height for Y+ = 30 is 0.92 [mm].
The reported value in CFX with uniform velocity inlet is 33.2
The reported value in CFX with uniform velocity inlet is 16.9, which is approximately half of the CFX as expected.
The reported value in CFX with 7th power velocity profile at inlet is 24.2.
The Y+ value tend to be higher in the developing region. Hence, Y+ value in flow with uniform velocity inlet is higher than that with velocity profile with power law typically observed in turbulent duct flows.
The calculation was repeated in CFX using velocity profile specified by exponential function V0*[1-(r/R)7] keeping all other parameters constant. The reported value of Y-plus has reduced from 33.2 to 24.2
Energy Spectrum of a Turbulence Flow: Big whorls have little whorls, which feed on their velocity; And little whorls have lesser whorls, And so on to viscosity.
The content on CFDyna.com is being constantly refined and improvised with on-the-job experience, testing, and training. Examples might be simplified to improve insight into the physics and basic understanding. Linked pages, articles, references, and examples are constantly reviewed to reduce errors, but we cannot warrant full correctness of all content.
The Y+ value is a dimensionless parameter that represents the distance from the first grid cell to the surface wall. Here, u_τ is the friction velocity, y is the wall distance, and μ is the kinematic viscosity of the fluid.
A value of 2 is typically OK although this might need to be as low as 1, especially when capturing boundary layer heat transfer. 10 boundary layers are typically recommended with SST. Also try changing the layer gradation, switching from auto to approximately 1.4. This can help to reduce the Y+ value.
Wall function is a good choice for a beginner, please make sure that Yplus is greater than 11 and lower than 300. For detailed information, please check the theory section in FLUENT help file. The rule of thumb is as follows: Use WALL FUNCTIONS when you know the boundary layer is always attached to the surface.
Y+ is the dimensionless distance normal to the wall. This is related to define mesh quality and helps in optimizing the mesh. People usually say that if Y+ is nearly equal to 1, mesh quality is very good and results can be accurate but it leads to highly refined mesh and obviously increase in computational cost.
For boundary layers in adverse pressure gradients, to correctly predict flow separation it is imperative to have a Y+ less than or equal to 1, meaning we are resolving the boundary layer flow all the way to the laminar sub-layer.
under Y Axis Function and select Wall Yplus from the drop-down list under that. Since we want the y+ value for cells adjacent to the wall of the pipe, choose wall under Surfaces. Click Plot. As we can see, the wall _y+_value is between 1.6 and 1.9 (ignoring the anomalous value at the inlet).
All Answers (19) Regarding the turbulence and Y+ with an enhanced wall function try to keep the Y+<5 and preferably ~1. this is to allow the enhanced wall function proper blending at the boundary layer. At least for two-phase turbulent flows.
The y+ value is a non-dimensional distance. It is often used to describe how coarse or fine a volumetric mesh is for a flow pattern. It is important for the modelling of turbulence to determine the proper size of the mesh cells near no-slip walls.
k-omega Models: The k-omega models, including the standard k-omega and the SST k-omega, are commonly used for complex flows. These models require a y+ value between 1 and 5 in the near-wall region. This ensures accurate predictions without using wall functions.
If Reynold's number lies below 5 × 105, then the flow of liquid is streamlined or laminar. If Reynold's number is greater than or equal to 5 × 105, the flow of liquid is turbulent.
The planetary boundary layer is the lowest layer of the troposphere where wind is influenced by friction. The thickness of the PBL is not constant. At night and in the cool season the PBL tends to be lower in thickness while during the day and in the warm season it tends to have a higher thickness.
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