Can't expand much I'm afraid - I'm an instructor with an interest in this stuff, not an aeronautical engineer. However, I'll try to clarify what I said. If a professional designer wants to correct my explanation, I'll not object!
I've never heard that straight wing aircraft exibit spanwise flow. From the tip to the root? An effect of changing chord or wash out? With constant chord and camber, if anything I'd think fuselage effects produce a slight spanwise flow towards the tips. With of course any winglet limiting this.
Straight wing aircraft have a limited degree of spanwise flow due to the circulation of air around the tip. Since air under the wing is at a slightly higher pressure than air over the wing, it will tend to circulate ("leak") around the tip. Air gets some momentum from this vortex, and tends to continue moving inwards towards the root over the upper surface of the wing. If there is washout, either by twisting the wing or by using a different wing section inboard from outboard, it may exacerbate this effect.
And when you talk of spanwise flow on swept wings of sin(sweep angle), I presume you're implying the airflow continues to move in the same direction of the aircraft's longitudinal axis. But surely on most swept wing aircraft there must be some lateral movement of the air, especially at high aoa, and towards the tips.
Yes, you're right. I suppose I didn't read the question carefully enough.
However, there is spanwise flow of the boundary layer, and it does vary with wing sweep at subsonic speeds. Partly it is driven by the leading edge of the swept wing, which tends to deflect air towards the tips (just as a vane held at an angle to the air flow will deflect the air).
Near the fuselage this effect is minor but near the tips it is quite significant - at subsonic speed, air reaching the leading edge near the tip has been affected by a pressure pulse from air reaching the wing root, which is closer to the front of the aircraft. Think of a parcel of air reaching the wing root - it shouts "hey, move up and move outwards"; this shout (pressure pulse) moves towards the tip at the speed of sound, with the result that the air near the tip has already started moving up and out before it reaches the wing.
A different way of explaining air movement is this: air over the wing will reach its lowest static pressure. It can move towards the trailing edge, an area of higher pressure, or it can be deflected a little along the wing span, a line of lower pressure. In practice it does a little of both, so air in the boundary layer tends to move along a swept wing to a much greater extent than an unswept wing.
Again, hope this actually makes sense in the cold light of day. There are lots of aerodynamics textbooks out there, and a very few that can explain these things without using formulae. I have a couple - at work, unfortunately, so I couldn't consult them in writing this!
Cheers,
O8