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Wikipedia:

"Mach number: the ratio of the airspeed to the speed of sound should be identical for the scaled model and the actual object (having identical Mach number in a wind tunnel and around the actual object is -not- equal to having identical airspeeds)" source

Why identical Mach number in a wind tunnel and around the actual object is -not- equal to having identical airspeeds?

If plane fly at 0.5 Mach(617km/h) that mean airflow speed for test model must also be 617km/h ,to has same compresibility effects? Isnt it?

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Why identical Mach number in a wind tunnel and around the actual object is -not- equal to having identical airspeeds?

The speed of sound depends on temperature, humidity, and pressure. It is highly unlikely that temperature, humidity, and pressure in the wind tunnel are exactly the same as the temperature, humidity, and pressure at the altitude the aircraft will be flying at.

If plane fly at 0.5 Mach(617km/h) that mean airflow speed for test model must also be 617km/h ,to has same compresibility effects? Isnt it?

Which altitude are you flying at? From the looks of it, you are talking about a car, not a plane!

At typical cruising altitudes for jets, 0.5 Mach is not 617 km∕h. At 10000 m, 0.5 Mach is 540 km∕h.

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