Better CPU

I believe that there's many factors influencing that question. Memory probably will have a key role as well.

Also licensing. Depending on your license it won't matter having more core's. 1-4 is one count in general. 5-8 is another..and 8-16 another.

Regarding the cpu's i'd guess the newer generation 10th family performs better, but again, processor only doesn't mean anything.

As Inspire Cast scaling is good for 4 cores, I would go for the the i7 with more frequency. As those 10 cores of i9 are unlikely to bring great benefit for Inspire Cast. Unless you want to run two jobs with 4 cores each. 

Which CPU will work faster?

i7-7800X @ 3.5GHz (6 cores) 

i9 10900 @2.8 GHz (10 cores) 

It doesn't make sense if you just look at the frequency and the number of cores.

Another factors to check: cache, supported memory type,...

At first shot i9 looks better, specially if a good cooling system is used.

Even if the original name looks like i7 specs has a higher clock, when you look into the details i9 specs might have a higher clock frequency if certain temperature thresholds are satisfed (and should be easier as it has less than 1/2 TDP).

What makes me doubt is the memory bandwith, as the i7 has 4 channels (but at inferior speed) and i9 has 2 channels, and it is known that memory bandwith has plays a major role in CFD type simylations.  I would say i9 is better, but I would not bet all my money on it.

This does not take into consideration the processor cost at all.

Now I am confused...

Dear Sourav, You say I9 is better, but no need for the 10 cores as it is 'using' 6 only?

Are there recommendations by Altair?

Hi Uri,

What do you mean '... beyond it remain same'?

I believe he means above 4 core's no considerable performance increase was found..just like convergence study.

So for this application don't waste resources as it will not leverage more tham 4 core's.

If you does not found any advantage about CPU time above 4 cores, your model is NOT BIG enough for real parallel computing.