Seems Flight Simulator is simulating winds by simply having constant wind in a direction.I think in real life, if there are 100kts winds, they sometime flow at 100kts then stop, then again, there is no constant wind. So a place flying with 100kts sidewind will move left/right all the time.Is that correct? or FS models winds correctly?

Well, no. FS itself is terrible at simulating winds aloft, but not bad for on the ground. AS is MUCH better since it takes RW data and plugs it in. On the ground, you can get gusty winds, but they don't blow hard one second then suddenly stop. Winds aloft are much more constant, but as to speed and direction, it depends on where in the world you are. Normally between 30* N/S and 60* N/S latitudes, the winds are westerly. Above 60* lat they tend to become northeasterly. Again, AS models that rather well, as on a flight from Northway AK to Prince Rupert, British Columbia yesterday at FL330, I started out with winds at 03025 (roughly - varied somewhat) and when I started my descent to YPR, they were 30085, which is pretty accurate. However, they won't go from being 30085 to calm. Upper level winds tend to be stronger, less gusty and somewhat less variable than surface winds. Of course, if you get in the jet, you can see winds up to and even beyond 200 kts (great tailwind if you are eastbound!).