Another Look At Slot Effect
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Another Look at Slot Effect Arvel Gentry puts the old theories to rest By Arvel Gentry SAIL Magazine July 1973 The slot effect as we know it is traditionally But in 1971 I obtained some results from a new andsupposed to do three basic things sophisticated computer program that indicated that the First the jib causes the air over the lee side of the main old explanations of how a wing slot works were 1999 December entirelyto have a much higher velocity increasing the partial wrong My results were accurate detailed and hence the sails efficiency by windtunnel data and very conclusive Second the higher velocity air in the slot revitalizes Does this mean that the old explanations for slot effectthe air over the main which would otherwise be in a were also wrong The answer is yesseparated or stalled condition My conclusion was not reached in haste for it is Third the increased velocity in the slot results because dangerous to say old ideas are wrong unless you have a lotthe distance between the leech of the jib and the main is of proof and can cover all the aspects of a problem To testmuch less than the distance between the headstay and the my theories I first made use of a device called an Analogmast but it must accommodate the same flow per unit of Field Plotter that determines the flow streamlines abouttime any airfoil combination Figure 2 I studied single and Sailing books magazine articles by national champions multiple airfoil combinations to gain a basicand our leading sailmakers all have expounded on and understanding of the directions the air takes as it flowsmade use of these ideas to tell us first how the slot works past the sailsand second to explain how we should trim our sails These A number of sail angles and relative positions of jibmainsail interaction originally were investigated and the results were then backed up by morederived from an description of how a detailed and accurate answers from the computer Next Iwing with a leading edge slot works Figure 1 conducted a series of experiments in a water channel to obtain photographic evidence of my findings On top of all this each new conclusion and explanation was subjected to the question do they all make sense in terms of actual sailing experience The answer continues to be yes In the first three articles of this series I presented a Figure 1 number of very important and fundamental aerodynamic principles In this fourth article I will assume you already Figure 2 Analog field plotter equipmenthave studied the first three parts However several items some of the air we would think might go through the slot isshould be repeated as a review actually diverted by the combined circulation fields so that In the April issue I pointed out that most of the airflow it goes on the lee side of the jibdiagrams in the sailing books violated some basic To examine the interaction between two sails we principles In the may issue we learned that use a typical airfoil section of a mainsail and a layer separation on a sail results when the section through the jib The actual airfoil shapes andsurface pressure is increasing too rapidly the airflow is angles are not too important as long as they are down too quickly And last month we learned representative of close hauled sailing conditions In factabout circulation and how lift is generated for any conclusion to be scientifically correct it must apply We saw that the airflow coming off both sides of the for almost any shape and for a wide range of angles and Ileech of a sail must be at the same pressure and speed the will illustrate the effect of different sheeting angles nextKutta condition The airflow speed will be higher than over the forwardlee portion of a sail but it will We always must check our results to see if a luffingreturn to near freestream speed by the time it reaches the condition would result This check will insure that theleech It is this slowing down of the leeside airspeeds and results are correct even though the analysis does make useresulting increase in pressures that causes the airflow to of rigid airfoils instead of flexible shapesseparate First look at the flow about the mainsail airfoil without From the information Ive presented so far it is quite the jib in place The mainsail will be positioned at the sameobvious that there are some serious angle it will have when the jib is in place the leading edgeabout the old sloteffect explanations in the sailing of the main is determined by the shape of the mast Forliterature But if previous explanations for the slot effect these studies the area right behind the mast was filled in toare wrong what are the right answers represent the separated region that always exists Until now Ive talked about general principles and immediately behind the mast Separation effects from theabout flow around a single sail Now well begin the mast are reasonably understood so I will not dwell on mastanalysis of two sails together The last article described effects herehow the flow about a lifting airfoil may be thought of as The calculated streamlines for a main alone are shownbeing the addition of a circulation flow solution and a non in Figure 4 Note that the stagnation streamline Sm solution The amount of circulation on the into the lower windward side of the sail The was adjusted so that the resulting airflow was in a pressure if no separation or stall occurs is shown in Figuresmooth direction off the leech But what about a case 5 Remember from last month that low pressures highwhere we have two airfoils velocities are represented by negative pressure Figure 3 shows that both the jib and mainsail have their coefficients and that high pressures low velocities areown circulation fields The strengths of the two shown as positive pressure must be adjusted so that the Kutta condition is Since the stagnation streamline comes in on thesatisfied at the leech of both sails smooth flow off each windward side of the sail upwash and we have a goodsail In Figure 3 note that the two circulation fields oppose pressure difference on the two sides of the surface the sailand tend to cancel each other in the slot between the jib will hold its shape and not luff Even though the forwardand main This fact gives us a hint that we will not get all lee part of the sail seems to be facing the freestream airflowthe increased air speed in the slot that is claimed by the old direction way out in front of the sail the upwash effecttheories places the leading edge of the sail at a higher angle so that it If the slot flow did give higher velocities that increase will not luff The sail only sees the local upwash flowthe partial vacuum on the main would not this same vacuum on the windward side of the jib reduce its Because the stagnation point is around on theefficiency As we will see later what really happens is that windward side of the airfoil we get a very high suction peak large negative pressures as the air tries to make the sharp turn around the mast to the lee side The pressure then starts to increase rapidly toward the pressure it must attain by the time the leech is reached to satisfy the Kutta condition The boundary layer probably will not be able to withstand this steep increase in pressure the flow will separate and the airfoil will be in a stalled condition To prevent this stall the sheeting angle of the sail is increased either by letting out the mainsheet or moving the traveler to leeward This is exactly what our experience is afloat when the jib is lowered However in the example shown in Figures 4 and 5 the mainsail is at the same angle Figure 3 at which it would be if the jib were present And the 2 Figure 4 Figure 5pressures shown in Figure 5 are what we get if the flow was used alone are also given the dotted lines A numberdoes not separate of important points come out of this figure Carefully note the shape and position of streamline H First the stagnation streamline for the mainsail whenin Figure 4 This line is selected so that it goes through the the jib is present the solid line Sm goes smoothly into thepoint H that will be the leading edge for the jib the leading edge of the mast instead of being down around onheadstay in our next example The distance between the the windward side as was the case for the mainsail streamline Sm and the headstay streamline H at The air therefore will not have to speed up as much to getthe left side of the figure is a measure of the amount of air around to the lee side of the main This means that thethat passes between the headstay and the mast without airspeeds will not be so high over the forwardlee side ofthe jib being present and without any separation on the the main and the flow will not have to slow down so muchmainsail to reach the final speed that is required at the leech to Now lets introduce the jib The streamlines when both satisfy the Kutta conditionthe jib and main are used are shown in Figure 6 This This gives a smaller increase in pressure as the air flowsfigure is necessarily a bit cluttered so that the flow lines toward the leech a lower adverse pressure gradient andbefore and after the jib is added can be compared helps prevent the flow from separating If the jib were Figure 6 is the most important figure in this whole placed in the picture and had it caused higher air speedsseries so study it carefully The solid streamlines represent over the forwardlee side of the main as the old venturithe flow when both the jib and main are used For explanation states then it would give a steeper the streamlines that existed when the main gradient and actually cause the flow to separate rather 3 Figure 6 Figure 7than prevent it point A Note that at this point it is exactly the same Second the streamline H that went through the distance away from the surface of the main as the dottedheadstay point when the mainsail was used alone now streamline is for just the main at point A This means that atgoes well above the surface of the jib The new streamline this point we have about the same airspeeds when wethrough the headstay the stagnation streamline for the have a jib and main as we had for the main alone In factjib Sj now is much lower than the headstay streamline H on the surface of the main itself Figure 7 shows slightlyin the case of the mainsail alone higher pressures less negative and therefore lower The distance between the two stagnation streamlines velocities when the jib is used than occurs without the jibSm and Sj at the left side of the figure is a measure of the From all this we have to conclude that the old venturi slotamount of air that now goes between the headstay and the effect explanation must be wrongmast and therefore into the slot between the two sails The calculated pressure distributions for the mainsailwhen both sails are present You can see that much less air both with and without the jib are shown in Figure 7 Thegoes between the headstay and the mast when both sails are presence of the jib and a resulting shift in the than it does when only the main is used Much more point on the mainsail causes a drastic reduction in the highair is being deflected around the lee side of the headstay over the forwardlee part of the mainand therefore around the lee side of the jib than when Since the pressure gradients are much lower thethere was a mainsail alone possibility of complete flow separation on the mainsail is Look closely at the solid streamline for the main and jib reduced and the amount of theoretical lift that passes in the slot between the two sails at contributed by the mainsail is also reduced 4 Of course in the case of the mainsail alone have occurred in real life and you would not havebeen able actually to realize the amount of lift when neglecting separation You do lose liftfrom the theoretical nonseparated value but you nowhave reduced pressure gradients so the airfoil will not stall These findings are the keys to the often of slot flow between sails With streamlines you can see that the air passingbetween the two sails is quite different from the oldventuri effect explanations many of us have grown upwith With both sails set a large percentage of the air thatwas going between the headstay and the mast when themainsail was alone now goes above and down the lee sideof the jib Less air is left to pass in the slot between the sailsand this tube of air actually slows down the out as it reaches the line between the headstay andthe mast Then and only then does it begin to speed back up as itapproaches the slot between the jib and the by the end of the slot the speed has back to about what it would have been at thatpoint if the mainsail alone was used These flow diagrams also verify a couple of points thatwe all have observed in actual sailing experience The jibreduces the upwash on the main gives the main a headerand the main increases the upwash a lifting wind shift forthe jib Thus we see that the primary effect of a jib is to causereduced velocities over the forwardlee part of the mainrather than increased velocities The slower velocities inturn give reduced pressure gradients that help and stall rather than some higher The velocities in the slot are determined by the totaleffects of the circulation fields around the two sailsnecessary to give smooth flow off the leeches the Kuttacondition The flow streamlines for this condition shouldnever be drawn by hand or guessed Instead they must beaccurately determined by an Analog Field Plotter orcomputer This month we have studied the main alone and thenadded a jib It is equally interesting to do just the oppositeto look at the airflow about the jib alone and then add themain This is what we will do next month 5
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