Dear ENVI-met,

1. Model basic information: grid 50 × 50 × 30, grid resolution is 10, 10, 2. The building is located in the center of the model and measures 300 meters by 300 meters by 25 meters.

2. Basic information of calculation: the summer wind speed is 2.8m / s (at a height of 10 meters), the initial wind direction is 202.5 °, and the ground roughness is 0.22.

3. The output result is expected:

V0 = 2.8 × (1.5 / 10) ^ 0.22 = 1.84m / s (initial wind speed at 1.5 meters)

Ri = Vi / V0 When Vi is 1.84m / s, Ri = 100% (the wind speed ratio of the initial wind speed at 1.5 meters)

4. Actual output:

1) When Ri is 100%, the corresponding wind speed is 2.27m / s instead of 1.84m / s;

2) When the wind speed at the height of 1.5 meters is 1.84m / s, the corresponding wind speed ratio is about 82% instead of 100%;

What is the reason for the difference? Does it have anything to do with nested grids?

Best wishes,

Pat

## About wind speed to wind speed ratio

### Re: About wind speed to wind speed ratio

Hi Pat,

Nesting grids of course influence your results. I don't understand your model area... where is the building? Did you cut the results in Leonardo?

Why don't you create a super simple model area without buildings and nesting grids for such analysis?

Best regards,

Tim

Nesting grids of course influence your results. I don't understand your model area... where is the building? Did you cut the results in Leonardo?

Why don't you create a super simple model area without buildings and nesting grids for such analysis?

Best regards,

Tim

### Re: About wind speed to wind speed ratio

Dear ENVI-met,

thank you for your advice. I created a super simple model area without buildings and nested grids to compare the wind speed ratio distribution map with the wind speed distribution map. The model related settings are in Appendix 1-3.

(1) V0 = V1 * (1.5m / 10m) ^ 0.22, V0 is the initial wind speed of the wind at a height of 1.5 meters, and V1 is the initial wind speed of the wind at a height of 10 meters. Therefore, when V1 is 2m / s, V0 should be 1.32 m / s.

(2) According to the wind speed ratio formula: Ri = Vi / V0, when Ri is 100%, Vi should be 1.32m / s.

However, from the two distribution maps of simulation results (4-5), when Vi is 1.32m / s, Ri on the graph is about 72%. What is the reason for this?

Thank you very much for your answers.

thank you for your advice. I created a super simple model area without buildings and nested grids to compare the wind speed ratio distribution map with the wind speed distribution map. The model related settings are in Appendix 1-3.

(1) V0 = V1 * (1.5m / 10m) ^ 0.22, V0 is the initial wind speed of the wind at a height of 1.5 meters, and V1 is the initial wind speed of the wind at a height of 10 meters. Therefore, when V1 is 2m / s, V0 should be 1.32 m / s.

(2) According to the wind speed ratio formula: Ri = Vi / V0, when Ri is 100%, Vi should be 1.32m / s.

However, from the two distribution maps of simulation results (4-5), when Vi is 1.32m / s, Ri on the graph is about 72%. What is the reason for this?

Thank you very much for your answers.

### Re: About wind speed to wind speed ratio

Hi,

I don't know why you have that strange wind speed distribution in your model area if it is empty, has no nesting grids and the wind is coming from 180°.

However, the calculation does not work because your z0 is too high. The simulation gives you an error for that as well: It reduces te z0 automatically to half the size of the lowest grid cell. Since you have 1 m resolution in Z and activated splitting it is quite low. I tested it and it was reduced to 0.09 for that configuration.

Best regards,

Tim

I don't know why you have that strange wind speed distribution in your model area if it is empty, has no nesting grids and the wind is coming from 180°.

However, the calculation does not work because your z0 is too high. The simulation gives you an error for that as well: It reduces te z0 automatically to half the size of the lowest grid cell. Since you have 1 m resolution in Z and activated splitting it is quite low. I tested it and it was reduced to 0.09 for that configuration.

Best regards,

Tim

### Re: About wind speed to wind speed ratio

Dear Tim,

Thank you for your answer, but I don't understand the "the calculation does not work because your z0 is too high" you mentioned. Except when the initial wind speed value is set at the height of 10 meters, the heights of the wind speed and the wind speed ratio map in Figure 4 and Figure 5 are 1.5 meters, so what is too high?

In addition, I also made a simulation with a building. The specific settings are shown in Figures a-e. There is still a problem that the wind speed and wind speed ratio cannot be verified by formulas.

When the wind speed at a point on the f1 chart is 1.3m/s at a height of 1.4 meters, the wind speed ratio at its location is normally calculated by the formula as 100%, but from the wind speed distribution chart f2, it is found that the wind speed ratio is 66.37% -79.22%. I still wonder where the problem is?

Thank you so much for your answers.

Pat

Thank you for your answer, but I don't understand the "the calculation does not work because your z0 is too high" you mentioned. Except when the initial wind speed value is set at the height of 10 meters, the heights of the wind speed and the wind speed ratio map in Figure 4 and Figure 5 are 1.5 meters, so what is too high?

In addition, I also made a simulation with a building. The specific settings are shown in Figures a-e. There is still a problem that the wind speed and wind speed ratio cannot be verified by formulas.

When the wind speed at a point on the f1 chart is 1.3m/s at a height of 1.4 meters, the wind speed ratio at its location is normally calculated by the formula as 100%, but from the wind speed distribution chart f2, it is found that the wind speed ratio is 66.37% -79.22%. I still wonder where the problem is?

Thank you so much for your answers.

Pat

### Re: About wind speed to wind speed ratio

Hi,

No, you entered a roughness length (z0) of 0.22 and calculated the estimated values with this roughness length. However, this value is too high for your chosen vertical resolution and thus automatically reduced to half the size of the lowest grid cell. It gives you an error message for that as well:

With 1 m resolution and splitting it was reduced to 0.09. With your new example in 2 m vertical resolution it is reduced to 0.18. The calculation behind it is 0.5 * dz[1] * 0.9.

So, you need to enter a lower roughness length which is not recalculated or take the values I just posted here.

Best regards,

Tim

No, you entered a roughness length (z0) of 0.22 and calculated the estimated values with this roughness length. However, this value is too high for your chosen vertical resolution and thus automatically reduced to half the size of the lowest grid cell. It gives you an error message for that as well:

Roughness legth z0: Maximum z0 is half the grid size dz of the lowest box.

I have set it to the maximum possible value

With 1 m resolution and splitting it was reduced to 0.09. With your new example in 2 m vertical resolution it is reduced to 0.18. The calculation behind it is 0.5 * dz[1] * 0.9.

So, you need to enter a lower roughness length which is not recalculated or take the values I just posted here.

Best regards,

Tim

### Re: About wind speed to wind speed ratio

Dear Tim，

I see, thank you very much for your careful answer！

Best wishes，

Pat

I see, thank you very much for your careful answer！

Best wishes，

Pat

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